The principle of the operation of the voltage transformer is shown in FIG. 1, which depicts two coils wound on the same core. The power supply 100 and core 103 are connected by the first coil, namely, primary winding 104. Similarly, the load 105 and the core 103 are connected by the second coil, namely, secondary winding 106. The voltage of the power supply 100 on the terminal a.sub.1 and b.sub.1 is v.sub.1, and the current flowing into the primary winding 104 is the primary current i.sub.1. The terminals of the secondary winding 106 are a.sub.2 and b.sub.2.
The primary current i.sub.1 in the core 103 induces the magnetic flux .phi., and then the secondary current i.sub.2 is induced by the magnetic flux .phi. in the secondary winding 106. Finally, the secondary current flows into the secondary winding 106, and the voltage drop between terminal a.sub.2 and b.sub.2 is thus produced. Assuming that the magnetic flux .phi. passes through an N-turn coil, an induced voltage, denoted v.sub.2, is related to the rate of change of the magnetic flux by v.sub.2 =N.sub.2 (d .phi./dt) and ignoring the energy loss in the core, the equation N.sub.1 i.sub.1 =N.sub.2 i.sub.2 is obtained. Thus, the principle of energy conservation there will be an equation i.sub.1 v.sub.1 =i.sub.2 v.sub.2 and the voltage drop between the terminals a.sub.2 and b.sub.2, denoted v.sub.2, related to the voltage drop between the terminals a.sub.1 and b.sub.1, is by equation (1), wherein: EQU (V.sub.1 /V.sub.2)=(N.sub.1 /N.sub.2) (1)
According to equation (1), the design and the manufacturing of the primary winding and the secondary winding must take the turn ratio (N.sub.1 /N.sub.2) into account. According to the structure of solenoidal winding used in the traditional transformer, the process for manufacturing the voltage transformer could become more complex when the high voltage transformer is considered because the number of turns in each coil is very large. Furthermore, the special design of the insulation between the layers of the coils must be considered. Due to the reason mentioned above, the manufacturing and the design of the traditional high voltage transformer are very complicated. In some application, the multiple circuit is used to raise the output voltage of the voltage transformer, and the configuration of the multiple circuit is shown in FIG. 2. The output voltage of the secondary side T.sub.xs is assumed to be V.sub.M, and the output voltage V.sub.M is fed into the multiple circuit to amplify the output voltage in every stage from the multiple circuit. Then, the output voltage of every stage of the multiple circuit can be fed into another voltage transformer to further increase the output voltage in high-power application.
In the case of switching-power-supply application, the frequency is usually between 100 kHz and 200 kHz, so the effect of the distributed capacitance must be considered carefully in the traditional high voltage transformer. In addition, the volume of the modern electronic device is shrunk to fit the need of the modern application, so the volume of the voltage transformer used in the power supply must be reduced too. Thus, the switching frequency of the power supply is increased, and the reduction of the volume of the transformer is then enabled. Furthermore, when the switching frequency is low, it is not important to ignore the parasitic effect because it is not obvious. Whereas, when the switching power is high, the consideration of the parasitic effect of the components of the transformer is critical, and the poor efficiency of the transformer as well as the modification of the feedback circuit of the transformer are very complicated.
In addition, the components used in the traditional voltage transformer are all specially designed. For example, rectifier and capacitor-use filter in high-power application must be utilized in high voltage transformer. So they are suitable to be used under high component stress. Due to the specialty, the source of the specially designed components have been a problem, and the high voltage transformer is thus very expensive.